This application is based upon and claims benefit of priority of Japanese Patent Application No. 2001-263120 filed on Aug. 31, 2001, the content of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to an alternator for use in an automotive vehicle such as a passenger car or a truck.
2. Description of Related Art
A voltage regulator is used for controlling an output of an automotive alternator. Some types of conventional voltage regulators include a function to detect turning-on of a key switch. Such a voltage regulator initiates supply of exciting current to an alternator upon detection of the turning-on of the key switch. It is usually required to provide signal lines for detecting the key-switch operation.
In order to detect the key switch operation without using the signal lines, JP-U-62-44698 proposes a technology for detecting initiation of rotation of an alternator based on a frequency of a voltage induced in an armature winding due to residual magnetism in field poles. In this manner, whether the alternator is started, i.e., whether the engine is cranked up is detected. Upon detecting the start-up of the alternator, exciting current is supplied to the field winding of the alternator. There has been a requirement, however, to increase a level of voltage induced by the residual magnetism because an amount of residual magnetism is small. U.S. Pat. No. 4,409,539 proposes to improve magnetic characteristics of the field poles by annealing the field poles. Further, JP-A-3-215200 and JP-A-8-503308 disclose a technology to detect initiation of the engine rotation based on a line voltage induced in armature windings of two different phases.
It is found out, however, that there are respective problems in those conventional technologies. In U.S. Pat. No. 4,409,539, though the residual magnetism is improved, a magnetic resistance increases and thereby the output of the alternator in its normal operation decreases. Accordingly, the alternator size has to be enlarged to maintain the output. Further, a large size furnace is necessary for performing the annealing, which results in manufacturing cost increase.
According to the technology disclosed in JP-A-8-503308, a resistor is connected to a phase winding which is different from a phase winding with which the induced voltage is detected, and the phase winding to which the resistor is connected is grounded through the resistor thereby to form a closed circuit. A negative potential also appears across the resistor in a negative half cycle. This means that a so-called line voltage in three-phase windings appears across the resistor. To detect the negative potential, a complex circuit for providing a negative reference potential is required, which results in manufacturing cost increase. Accordingly, only a positive potential appearing in a positive half cycle is practically detectable. The voltage appearing in the positive half cycle cannot be amplified in the technology disclosed in this publication. Therefore, a detecting threshold has to be lowered to improve detection accuracy. If the detecting threshold is lowered, however, a signal-noise ratio (S/N ratio) becomes low because of influence of a leakage current flowing into the alternator from an on-board battery. As a result, it becomes difficult to accurately detect the signal indicating initiation of rotation. U.S. Pat. Nos. 5,376,876 and 6,147,474 propose to eliminate the influence of the leakage current by changing the threshold level according to an armature voltage. To change the threshold level, however, it is necessary to add a logic circuit for this function.
In the device disclosed in JP-A-3-215200, a potential difference between two phases is detected by floating it from the ground. There is a possibility, however, that the detecting circuit is damaged due to a large amount of current flowing into the detecting circuit when the alternator begins to generate power. It is necessary to provide a protecting circuit, such as a current limiter or a switching circuit for intercepting the current, to avoid the above problem. It is not advantageous to provide such a protecting circuit because it makes the detecting circuit complex and expensive. Especially, when the switching circuit is employed, it is required to provide the switching circuit with a proper current capacity and counter-measure against a surge voltage generated by intercepting the current.
The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide an improved alternator in which initiation of the alternator rotation is properly detected by a simple structure without using signal lines for detecting turning-on of a key switch.
The alternator includes a rotor, a stationary armature, a rectifier and a voltage regulator. The rotor includes plural field poles and a rotor winding to which excitation current is supplied. The armature has multi-phase armature windings, such as three-phase windings. Alternating current is generated in the armature in a rotating field of the rotor. The rectifier rectifies the alternating current into a direct current that charges an on-board battery. The voltage regulator controls the output of the alternator by controlling excitation current supplied to the field winding.
In order to detect initiation of the rotor rotation, without using signal lines for detecting operation of a key switch, a resistor is connected to one of the phase-windings, e.g., a Y-phase winding. Upon initiation of the rotor rotation, a voltage induced by residual magnetism without supplying the excitation current appears across the resistor. A level of the induced voltage, however, is low because the amount of the residual magnetism is small. To increase the voltage across the resistor to a sufficient level to accurately detect the initiation of the rotor rotation, a capacitor is connected between another phase-winding, e.g., a Z-phase winding, and a negative terminal of the rectifier. The capacitor charged in a negative half cycle period is discharged in the following positive half cycle period thereby to increase the voltage across the resistor.
The voltage across the resistor is compared with a threshold voltage, or a reference voltage. It is determined that the rotor rotation is initiated when the voltage across the resistor becomes higher than the threshold voltage. Since the voltage across the resistor is increased to a sufficient level, the initiation of the rotation is accurately detected without lowering the threshold voltage level. Upon detection of the initiation of rotation, the excitation current starts to be supplied to the field winding, and thereafter the excitation current is controlled by the voltage regulator to maintain the output of the alternator at a desired level.
Preferably, a frequency of the voltage across the resistor is compared with a reference frequency in addition to detection of the voltage level, and initiation of the rotor rotation is detected when the voltage level reaches the threshold and the frequency exceeds a predetermined frequency. There is a possibility that the voltage level may be affected by leakage current flowing from the on-board battery to the resistor through the armature windings. Initiation of the rotor rotation is more accurately detected by detecting both the voltage level and the frequency. Preferably, an impedance of the capacitor in a frequency region corresponding to an idling speed of the automotive engine is set to a level lower than a resistance of the resistor. In this manner, the capacitor can be sufficiently charged thereby to increase the voltage level across the resistor.
The rectifier may be constituted by Zener diodes. In this case, the impedance of the capacitor is set not to raise the voltage across the resistor to a level exceeding a Zener voltage (a breakdown voltage in the inverse direction).
According to the present invention, the initiation of rotation is accurately detected by a simple circuit structure without using signal lines for detecting the key switch operation.
Other objects and features of the present invention will become more readily apparent from a better understanding of the preferred embodiment described below with reference to the following drawings.